1. Field of the Invention
The present invention relates to a liquid crystal composition and a liquid crystal display device having bistability, and especially relates to a ferroelectric liquid crystal composition and a ferroelectric liquid crystal display device.
2. Description of the Prior Art
These days, liquid crystal display devices have been widely used for both consumer equipment and office automation equipment such as portable TVs and laptop computers or the like, and now are successively replacing the CRT display devices. At present, the STN liquid crystal of double layer type utilizing a nematic liquid crystal is a main stream in the related market, but it has many problems with regard to response time, angle of view, and display capacity. On the contrary, ferroelectric liquid crystal has some excellent characteristics such as quick response and a large memory storage or the like which can not be found in the nematic liquid crystal. So, research and development about the ferroelectric liquid crystal have been actively conducted in order to produce a display of the next generation of large display capacity and with high definition.
FIG. 1 shows a schematic view of the ferroelectric liquid crystal. The ferroelectric liquid crystal has a layer structure usually called a smectic liquid crystal, and its liquid crystal molecules have such a structure that the molecules are tilted in the direction of a layer normal at a tilt angle .theta.. In FIG. 1, numeral 1 indicates a ferroelectric liquid crystal molecule, 2 indicates a cone, 3 (P) indicates a spontaneous polarization, 4 indicates a layer structure, and Z indicates a direction of a layer normal.
As shown in FIG. 1, the ferroelectric liquid crystal molecule can move freely on the cone (conical shape) titled in the direction of the layer normal at .theta. degrees, and longitudinal directions of the molecules are specific in each of the layers, so that the molecules as a whole have a twisted configuration.
The ferroelectric liquid crystal molecule is an optically active liquid crystal molecule which is not a racemic modification, and has the spontaneous polarization 3 running in a direction perpendicular to the longitudinal direction of the molecule. Accordingly, the molecule rotates back and forth around the layer normal in 2.theta. degrees alternatively depending upon the direction of an applied electric field. According to these two states of the molecules, either the bright or the dark can be displayed with a polarizer.
FIGS. 2A-2H show display modes of the ferroelectric liquid crystal and changes of brightness. A numeral 5 indicates an upper surface of a glass substrate, and 6 indicates a lower surface of the glass substrate. In FIG. 2A, the electric field is applied in the direction from the lower substrate toward the upper substrate and all the spontaneous polarizations extend upward uniformly. In FIG. 2B, the electric field is applied from the upper substrate to the lower substrate, and all the spontaneous polarizations extend downward uniformly. Each state of the molecules will remain even if the electric field is cut off. As shown in FIGS. 2A and 2B, idealistically perfect is such a switching between two uniform states in which all the long axis 1 of the molecules arranged from the top to the bottom extend in the same direction, but in practice the long axes of the molecules at the upper surface and at the lower surface are in a twisted position with each other by 2.theta. degrees as shown in FIGS. 2C and 2D affected by alignment layers and liquid crystal materials or the like, and the molecules therebetween are twisted with one another. This state is called `twist state`. There are thus a switching between a twist state and another twist state, and another switching between a uniform state and a twist state, and in addition there is also a monostable state in which one uniform state becomes stable. FIG. 2E shows applying voltages and changes of the brightness at the switchings between the uniform states, FIG. 2F shows those between the twisted states, FIG. 2G shows those between the uniform state and the twist state, and FIG. 2H shows those at the monostable state. "o" shows the brightness just at applications of an electric field, and "x" shows the brightness during states of being stored. The brightness during the memory state is important at a practical working. In FIG. 2E, the sufficient changes of the brightness can be achieved both at the voltage applications and during the memory state, but the changes of the brightness during the memory state are not sufficient at both sides in FIG. 2F and at one side in FIG. 2G, and the brightness never changes in FIG. 2H. As described just above, the twist state and the monostable state prevent sufficient changes of the brightness and can not realize a favorable contrast.
There have been the following three problems about the conventional ferroelectric liquid crystal panel. The first is that it hardly has the switchings between the desirable uniform states and tends to have the switchings between the twist states or between the twist state and the uniform state so that it is difficult to achieve favorable contrast. The second is that a deterioration of threshold characteristics occurs after leaving it in the memory state, namely one uniform state, for a long time. The third is that a reset pulse having a pulse width wider than a selecting pulse has to be applied when a matrix waveform is applied so that its picture plane becomes dazzling. The reasons for this phenomenon have not been found yet, but it is assumed to be because of ionic impurities. As for controlling the ionic impurities, there are examples (Japanese Laid-Open Patent Publications No. 52-108385 and 54-82388) in which a variety of additives were added to a liquid crystal in order to prevent a deterioration of the liquid crystal caused by DC applications, more than ten years ago when a DSM (dynamic scattering mode) was performed utilizing the nematic liquid crystal. But at present day the ions do not have to be controlled in a TN mode and an STN mode utilizing the nematic liquid crystal because the liquid crystal is purified sufficiently and is driven only by AC. A TFT presently used in a liquid crystal TV or the like has a problem concerning a voltage retention time, so that there is an example (Japanese Laid-Open Patent Publication No. 62-173438) in which minute particles which have absorptivity are added to the liquid crystal in order to improve the voltage retention time, but it has a deterioration system different from that of the ferroelectric liquid crystal and this example is not effective for the ferroelectric liquid crystal.
As for controlling the ionic impurities in the ferroelectric liquid crystal, there has been a trial (Japanese Laid-Open Patent Publication No. 63-163426) to remove the ionic impurities by adding an inorganic metal to the liquid crystal. But the addition of these minute inorganic metallic particles brings about a dispersion of the metallic particles in the liquid crystal, and causes problems such as a disorder of a molecular alignment of the liquid crystal, a tendency that a crystallization can easily occur when the crystal remains at a low temperature using the particles as cores for the crystallization, and lowering of resistivity, so that the ionic impurities can not be controlled effectively and it is difficult to keep favorable characteristics.
In order to prevent the deterioration of the threshold characteristics of the ferroelectric liquid crystal panel, there is another trial (U.S. patent application Ser. No. 07/422,493 filed on Oct. 17, 1989) in which a compound of a primary amine and of a secondary amine or an epoxy compound are added to the crystal, and this method can effectively prevent the deterioration. But there are still possibilities of having the twisted state because of its high reactivity.